Conventionally, communications that use shortwave band having frequencies from 3 MHz to 30 MHz are known. Such communications using the shortwave band have characteristics of reaching long distances by manipulating reflection with respect to the ionosphere and the earth surface (see, for example, Japanese Laid-Open Patent Publication Nos. 2005-333291 and 2003-234683).
However, due to the constantly changing state of the ionosphere, radio waves may pass through the ionosphere without being changed by the ionosphere or reflected from the ionosphere. Further, the radio wave may attenuate according to the state of the ionosphere. Further, the ionosphere exhibits different behavior with respect to radio waves according to the frequency of the radio waves.
Therefore, communication using the shortwave band is unstable due to the influence of the constantly changing state of the ionosphere. Thus, the transmission quality of communication using the shortwave band is desired to be improved.